The present invention concerns a positive electrode for use in lithium cell and a lithium cell using the same.
As electrode materials for lithium secondary cell, it is generally to use carbon compounds such as graphite for a negative electrode and a composite oxide as an electrode active material for a positive electrode. Such positive and negative electrodes are manufactured, for example, by slurryfying an electrode active material, a binder and a electrically conducting agent such as conductive carbon used optionally for improving the electron conductivity with a dispersion medium to prepare a paste or a coating solution, coating the same to an electrode collector by a coater and evaporating the dispersion medium.
An organic dispersion medium such as N-methyl-2-pyrrolidone has been used as the dispersion medium for manufacturing the coating solution but it involves a problem of toxicity in the manufacturing premise, a problem in view of safety, effects of the dispersion medium wastes on environments, a problem of processing cost, etc. In view of the above, it has been demanded to use water which reduces manufacturing cost and does not cause such problems.
In the manufacture of the negative electrode, use of water has been enabled recently as a dispersion medium by using a synthetic rubber latex type adhesive and a viscosity improver, and a manufacturing method capable of avoiding the problems described above has become popular.
On the other hand, in the manufacture of the positive electrode, while LiCoO2 has been widely used generally as the positive electrode active material, when water is used as the dispersion medium upon preparing a positive electrode by a positive electrode paste using LiCoO2 as the positive electrode active material, this results in a problem that LiOH, etc. are formed by reaction of lithium ion in the active material and water to cause corrosion of a collector and deteriorate the cell performance. Accordingly, it is still customary at present to use a styrene butadiene rubber or polyvinylidene fluoride (PVDF) for the binder and a non-aqueous dispersion medium such as N-methyl-2-pyrrolidone for the dispersion medium.
For solving the problem such as corrosion of the collector and enabling the use of water as the dispersion medium, JP-A No. 2000-294252 describes a positive electrode formed by coating the surface of a collector with a protective film of an oxalate salt or a compound of elements selected from silicon, chromium, and phosphorus. Further, JP-A No. 2003-157836 proposes to coat the surface of the positive electrode active material with a protective film having electroconductivity.
However, the positive electrodes described above are not still sufficient in view of the cell performance and since they require a step of covering the collector, etc., they involve a problem that they are disadvantageous also in view of the cost.
On the other hand, the present inventors have noted on lithium phosphate materials such as LiFePO4 as the positive electrode active material. Use of lithium phosphate in the organic dispersion medium system as the positive electrode active material is shown, for example, in JP-T No. 2000-509193, JP-A Nos. 9-134724, 2004-55493, etc.
When the present inventors have attempted to manufacture of a positive electrode by using lithium phosphates such as LiFePO4 in an aqueous system, it has been found that the problem of the corrosion for the collector less occurs. However, it has been found that the positive electrode active material agglomerates upon preparing the paste, tending to cause unevenness on the paste coated surface.
The present invention has been achieved in view of the above and it intends to provide a positive electrode for use in a lithium cell excellent both in electric characteristics and physical properties and a lithium cell using the same, by manufacture from an aqueous coating solution using safe water as a dispersion medium instead of organic dispersion media used so far thereby solving the problem of toxicity in the manufacturing premise, the problem of safety, and the problem of the effects of dispersion medium wastes on environments and the problem of the processing cost, as well as solving the problem of degradation of the cell performance caused by corrosion etc. of collectors and the problem for the occurrence of unevenness on the coated surface.